Though battling an injury, Serena Williams uncharacteristically lost a lead late in her quarterfinal loss to Sloane Stephens. / Vince Caligiuri, Getty Images

by Douglas Robson, Special for USA TODAY

by Douglas Robson, Special for USA TODAY

As her peers struggled with shaky forehands, jangled emotions and tortured tosses in big matches, Williams could always be counted on to finish the deal.

She was the closer, the woman who didn't hesitate to come up with the untouchable serve, dig out the desperate (and increasingly audible) winner, or happily keep her foot on the air hose until her opponent gasped.

But oddly, a period in which the 31-year-old American has reasserted herself as a dominant force - Williams is the reigning Wimbledon, U.S. Open and Olympic champion and hadn't lost since August - the sport's best competitor has dabbled with uncharacteristic bouts of competitive self-doubt.

Exhibit A: her stunning defeat to 19-year-old Sloane Stephens in the Australian Open quarterfinals Wednesday.

Hobbled by back spasms in the second set, Williams nonetheless allowed the adrenaline to kick in and put herself in a winning position. The 15-time Grand Slam champion led Stephens, who had never reached this stage of a major, by a break deep in the final set.

Williams then dropped the last three games to lose 6-3, 5-7, 6-4.

The power on her serve and backhand were diminished. But it was obvious from Williams' facial contortions, self-exhortations and one impressive racket demolition in the third set that she was struggling with more than her body.

"Absolutely, she was battling herself in third set," said U.S. Fed Cup captain Mary Joe Fernandez, who called the match for ESPN. "There's no question about it."

Williams has been there before - and more than most might imagine in the last few months.

Williams was two points from victory nine times but couldn't wrap it up in a shocking first-round exit at Roland Garros to 111th-ranked Virginie Razzano.

At Wimbledon, she was cruising past Agnieszka Radwanska of Poland but had to fight off her nerves and regroup in a tight 6-1, 5-7, 6-2 championship match victory.

In the U.S. Open final, she mowed through the first set but needed a late, four-game surge to defeat Victoria Azarenka 6-2, 2-6, 7-5.

"Everyone can see sometimes I'm uptight and not relaxing, which leads me not to perform my best," Williams said in an interview with USA TODAY Sports late last year.

"I feel like I got uptight at Wimbledon for sure," she added. "I don't really feel I got uptight at the U.S. Open. I mentally left."

On Wednesday following her loss to Stephens, Williams said her mentality in the third set was to lock down and play as if she were fine.

"At that point you just have to pretend like nothing's wrong," she said.

Sasha Bajin, Williams' long-term hitting partner, said the emotional management issues were minor, though he felt that Williams should have closed out Razzano in Paris.

"The French Open - she just wants it too much a little bit," said Bajin of the only major Williams has won once. "And that shows that she's just human. It happens."

Fernandez suggested a confluence of events - being the heavy favorite to win the tournament, competing against a young American that had recently played her close, feeling she should blow away any competition - conspired to unnerve her Wednesday.

"Even though she was up I felt she was stressed in the match the whole time," Fernandez said. "I felt she came out swinging but even though she was ahead she backed away from it. She got really cautious."

Added Fernandez: "When she's at her best she knows it. She's a level above everyone else. She's not used to often playing people that get close to her."

Chasing history can weigh on a player, too, and some have struggled with nerves as they aged.

Credit, too, must go to 29th-seeded Stephens.

Stephens, the daughter of NFL player John Stephens (now dead), will soon reside in the top 20 and is the first American teen into a Slam semifinal since 2001. She faces No. 1 Azarenka on Thursday.

"You have to give her credit for closing it out," Bajin said.

If the killer instinct that has defined her success at times betrayed her last year, the good news is that Williams also showed an ability to manage her nerves, hang on, and take control when her opponents wavered.

"It's really important for me to stay calm and do the right things," she said.

And when she's on, she can still blow away the field, as she did in crushing all comers at the Olympics.

"Champions always manage to find a way, whether they are playing their best, whether they are stressed or not," Fernandez said. "Not every day, but most of the time they do."

The question for Williams is whether she can learn to live with a career window closing and a perpetual crop of young players snorting down her neck.